More II documentation and examples

Hi folks! Just a quick heads up that we’ve overhauled the II README and added a simple example that you can just copy to get a working II integrated in your own projects. If you’re curious about how II works, or if you always wanted to write a canister with authentication, now’s the time!

If you have a look, I’d be very thankful if you could share some feedback here or through GitHub tickets/PRs, especially if things aren’t clear.